A common method of thermoforming plastic parts is compression molding. Compression molding usually uses solid molding tools, such as epoxy or composite materials cast around a copper cooling line configuration. Compression molding is used for thermoforming of sheet material, and is also used for attaching a cover material to a sheet article as the sheet article is thermoformed.
A significant problem experienced in compression molding is that epoxy or composite cast molds generally include a variety of hot spots on the mold surface which are spaced away from the copper cooling lines as a result of the configuration of the part being formed, or other tool design restrictions. These hot spots in the compression mold require significant cooling time for heat to be drawn away from the mold surface into the nearest cooling lines, which may substantially increase cycle time per part.
Another problem with such structures is that significant tool building time is required, as well as significant costs in preparation of the tool. Also, utilities, such as electricity, and support equipment, such as large hydraulic/electrical presses with water cooling equipment and controls, are required.
Accordingly, it is desirable to provide a method and apparatus for thermoforming sheet articles in which cycle time is reduced, tool preparation time is reduced, tooling costs are reduced, and capital equipment, support equipment and utilities costs and requirements are also reduced.
Compression molding is also used to make headliners to cover the interior surface of a motor vehicle roof. The press used to compress the mold halves together, however, is relatively large and expensive. Because the mold halves are relatively thick and require a significant amount of time to cool, moreover, cycle time per part is increased. The mold halves are also relatively heavy and expensive to manufacture.
U.S. Pat. No. 4,327,049 discloses another method of making headliners which includes compression molding multiple layers of pre-heated materials in a mold having two mold halves. These mold halves include a plurality of passages through which coolant fluid is routed to adequately cool the mold. Consequently, this method involves significant tooling and equipment costs.